>>1840338I'm sorry for being that guy but... if x is an item and U is a set then you can't make the state x ⊆ U unless you write it {x} ⊆ U, that is, that the set consisting in x is a subset or is equal to the set U. To use x as an item, it should be written x ∈ U, meaning that x is an element of U.
